Abstract

Herbivores mediate the abundances of primary producers both from the top-down, by consuming them, and from the bottom-up, by recycling nutrients. Whereas the top-down effects of herbivores on algae in marine ecosystems are well-documented, less is known about their roles as mediators of local-scale nutrient availability. We conducted a series of surveys and measurements of tide pools and the grazers in those pools between October of 2016 and June of 2017 at an intertidal site on the coast of Southern California, USA (33°35′16.3″N, 117°52′1.5″W). We surveyed grazer abundances in the field, measured biomass of representatives from four different grazer groups (littorine snails, limpets, chitons, and turban snails), measured ammonium excretion rates, and quantified ammonium accumulation rates in tide pools at our study site. We found that different grazer groups were characterized by different per-biomass ammonium excretion rates. Some grazer groups—turban snails and chitons—contributed more ammonium than predicted by their biomass, whereas other grazer groups—littorine snails and limpets—contributed less ammonium than predicted by biomass. Because of these differences between grazer groups, ammonium accumulation rates in tide pools at our study site were effectively predicted based on the ammonium excretion rates of the different grazer groups. However, ammonium accumulation rates were not related to total herbivore biomass. Our results highlight the importance of grazer identity—and particularly the role of species such as turban snails that contribute disproportionately to nutrient recycling—in understanding the contributions of grazers as mediators of bottom-up processes in marine systems.

Notes

Acknowledgements

We thank S. Bedgood, L. Elsberry, R. Fales, and B. Nguyen for field assistance and the City of Newport Beach for access to our study location. Funding was provided by the National Science Foundation (OCE 1736891 to M. Bracken and A. Martiny) and the UC Irvine Undergraduate Research Opportunities Program. We greatly appreciate the comments provided by P. Kraufvelin and two anonymous reviewers.

Funding

This research was made possible through funding provided by the University of California, Irvine, including an Undergraduate Research Opportunities Program award to J. Oates and A. Badten and a National Science Foundation Grant to M. Bracken and A. Martiny. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. All applicable state and institutional guidelines for the care and use of animals were followed. All collections were made under California Department of Fish and Wildlife Scientific Collecting Permit SCP-13405.

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